I don't understand why I have an error in the first call. I probably screwed up syntax in the second call, but the first call has no conversion. it calls an external C function passing an external C variable.

wxString can be implicitly cast to const char* but not to char*. You can either redefine the set_tuner function to take a const char* as the first paramenter or, if your sure the sure the string is not changed, use a const_cast to explicitly convert the wxString.